linux-kernel - Re: [PATCH v4 3/4] locking/qspinlock: Add ARCH_USE_QUEUED_SPINLOCKS

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Date:   Wed, 31 Mar 2021 09:12:05 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Guo Ren <guoren@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-csky@...r.kernel.org,
        linux-arch <linux-arch@...r.kernel.org>,
        Guo Ren <guoren@...ux.alibaba.com>,
        Will Deacon <will@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        Waiman Long <longman@...hat.com>,
        Anup Patel <anup@...infault.org>,
        Sebastian Andrzej Siewior <sebastian@...akpoint.cc>
Subject: Re: [PATCH v4 3/4] locking/qspinlock: Add ARCH_USE_QUEUED_SPINLOCKS_XCHG32

On Wed, Mar 31, 2021 at 8:44 AM Guo Ren <guoren@...nel.org> wrote:
> On Wed, Mar 31, 2021 at 12:18 PM Guo Ren <guoren@...nel.org> wrote:
> > On Tue, Mar 30, 2021 at 3:12 PM Arnd Bergmann <arnd@...db.de> wrote:
> > > On Tue, Mar 30, 2021 at 4:26 AM Guo Ren <guoren@...nel.org> wrote:

> > > As I understand, this example must not cause a deadlock on
> > > a compliant hardware implementation when the underlying memory
> > > has RsrvEventual behavior, but could deadlock in case of
> > > RsrvNonEventual
> > Thx for the nice explanation:
> >  - RsrvNonEventual - depends on software fall-back mechanisms, and
> > just I'm worried about.
> >  - RsrvEventual - HW would provide the eventual success guarantee.
> In riscv-spec 8.3 Eventual Success of Store-Conditional Instructions
>
> I found:
> "As a consequence of the eventuality guarantee, if some harts in an
> execution environment are
> executing constrained LR/SC loops, and no other harts or devices in
> the execution environment
> execute an unconditional store or AMO to that reservation set, then at
> least one hart will
> eventually exit its constrained LR/SC loop. *** By contrast, if other
> harts or devices continue to
> write to that reservation set, it ***is not guaranteed*** that any
> hart will exit its LR/SC loop.*** "
>
> Seems RsrvEventual couldn't solve the code's problem I've mentioned.

Ok, got it.

        Arnd